You can think of the PTR record as an opposite of the A record. While the A record points a domain name to an IP address, the PTR record resolves the IP address to a domain/hostname.
PTR records are used for the reverse DNS (Domain Name System) lookup. Using the IP address you can get the associated domain/hostname. An A record should exist for every PTR record.
The usage of a reverse DNS setup for a mail server is a good solution. Some external mail exchange servers make reverse DNS lookups before accepting messages originating from your mail server.
You can check whether there is a PTR record set for a defined IP address. The syntax of the commands on a Linux OS are:
dig -x IP
or
host IP
There can be only one PTR record per IP address. Multiple PTR records can not be set for one IP.